Alpha-alumina flakes

ABSTRACT

The present invention relates to alumina flakes having a defined thickness and particle size distribution and to their use in varnishes, paints, automotive coatings printing inks, masterbatches, plastics and cosmetic formulations and as substrate for effect pigments. and organic dyes.

The present invention relates to α-Al₂O₃ flakes and to the use thereof in paints, industrial coatings, automotive coatings, printing inks, cosmetic formulations and in particular as transparent substrate for effect pigments.

Imparting a pearlescent luster, metallic luster, color flop or multicolor effect can be achieved by using pearlescent pigments based on natural or synthetic transparent flakes. Pearlescent pigments based on α-Al₂O₃ flakes are well-known in the literature and commercially available under the trademark Xirallic® from Merck.

The important factors of a substrate for an effect pigment are the particle size, shape, surface property, refractive index and the like. Since large and small particles have different proportions of reflection and transmission of light on the particle surface, uniformness in particle size is essential for a vivid and uniform color. Also the particle size greatly affects the coloration of the pearlescent pigment because it is closely related with the wavelength of the light. That is, the smaller the particle size, the larger the surface area, thereby increasing the coloration and enhancing reflectivity, and offering a more vivid color. However, in coating metals or metal oxides on the surface of the Al₂O₃ flakes, it is usually not easy to provide uniform coatings on them and thus results in a decrease in the aspect ratio, which then reduces the effect of light interference thus deteriorating glossiness of the resulting pearlescent colors.

α-Al₂O₃ in the form of hexagonal flakes having a particle diameter greater than 10 μm and an aspect ratio (particle diameter/thickness) of 5-10 is known from Japanese Patent Publication No. 111239/1982.

The Japanese Patent Publication No. 72572/1991 discloses α-Al₂O₃ in the form of flakes having an average particle diameter of 0.5-3 μm.

The Japanese Patent Publication No. 39362/1992 describes Al₂O₃ in the form of fine platy particles of a hexagonal crystal system with the plane perpendicular to the c axis grown into a plate.

Al₂O₃ flakes composed of aluminum oxide (as a major constituent) and of titanium dioxide (as a minor constituent) are disclosed in U.S. Pat. No. 5,702,519. The Al₂O₃ flakes have an average particle diameter of about 5-60 μm, a thickness less than 1 μm, and an aspect ratio of >20.

WO 2006/101306 A1 and WO 2008/026829 A1 relate to zinc doped Al₂O₃ flakes and to pearlescent pigments based on these Al₂O₃ flakes. The Al₂O₃ flakes have an average thickness of 0.5 μm or less and an average particle diameter of ≧15 μm and a large aspect ratio (diameter/thickness) of ≧50 μm. These zinc doped Al₂O₃ flakes are not stable under acidic conditions and thus not suitable for all applications.

The Al₂O₃ flakes of the prior art have the disadvantages that they do not have a high chemical stability and/or have not the desired smoothness for the use of the flakes in cosmetic and paint applications.

The object of the present invention is to provide improved Al₂O₃ flakes having at the same time a high chemical stability, a smooth surface and high whiteness.

Surprisingly, it has now been found that the properties of the alumina flakes as such and of effect pigments based on alumina flakes can be increased by using alumina flakes with precisely defined dimensions and particle size distribution. Especially the optical properties of the alumina flakes and the effect pigments based on alumina flakes can be influenced by altering the particle size distribution.

Thus, the present invention relates to transparent alumina flakes which are distinguished by the fact that they have a thickness of ≧500 nm and a D₉₀-value of 30-45 nm and a D₅₀-value of 15-30 μm.

Compared to the prior art, the Al₂O₃ flakes according to the present invention show improved optical properties, in particular by their shimmer or glimmer effect combined with an increased chroma and luster and at the same time show a high chemical stability in all applications.

Since the coated and uncoated Al₂O₃ flakes according to the invention combine high gloss with intense interference colors and highly pronounced glitter effects, particularly effective effects can be achieved with them in various application media.

The alumina flakes according to the invention are used, in particular, as substrate for effect pigments, especially for the use in industrial applications. However, they can also be employed in all formulations where alumina flakes are usually employed, such as, for example, in inks, coatings, preferably automotive coatings, plastics, cosmetic formulations and as substrate for effect pigments.

The Al₂O₃ flakes of this invention have a particle size distribution characterized by a Gaussian distribution in which the volume size fractions are distributed as follows:

-   -   D₅₀ is in the range of 15-30 μm, preferably 15-25 μm     -   D₉₀ is in the range of 30-45 μm, preferably 30-40 μm.

In this patent application D₁₀, D₅₀ and D₉₀ of the alumina flakes are evaluated by using Malvern MS 2000.

The particle size distribution D₅₀ is also known as the median diameter or the medium value of the particle size distribution, it is the value of the particle diameter at 50% in the cumulative distribution and is one of the important parameter characterizing the particle size of pigments.

Correspondingly, the D₉₀ value indicates the maximum longitudinal dimensions of the Al₂O₃ flakes, as determined again by means of laser granulometry in the form of sphere equivalents, which 90% of the particles attain at maximum, or fall below, out of the entirety of all Al₂O₃ particles.

In a preferred embodiment, the Al₂O₃ flakes according to the present invention have a standard deviation of thickness distribution of less than 80, preferably 5-60 and in particular 10-50.

In a preferred embodiment the D₁₀ value of the alumina flakes according to the present invention is <9.5, preferably 9.0.

The D₁₀ value indicates the value of the longitudinal dimension of the Al₂O₃ flakes, as determined by means of laser granulometry in the form of the sphere equivalent, which 10% of the flakes attain at most, or fall below, out of the entirety of all the Al₂O₃ flakes.

The average thickness is determined on the basis of a cured paint film in which the Al₂O₃ flakes are oriented substantially plane-parallel to the substrate. For this purpose a transverse section of the cured paint film is examined under a scanning electron microscope (SEM), the thickness of 100 Al₂O₃ flakes being ascertained and statistically averaged.

The desired size and thickness distribution can be obtained by suitable classification of the flakes, such as by classifying through selected screens and the like.

The Al₂O₃ flakes according to the invention have a thickness of ≧500 nm, preferably 550-1000 nm, in particular 600-900 nm.

The Al₂O₃ flakes according to the invention preferably have an aspect ratio (diameter/thickness ratio) of 15-60, in particular of 20-45.

In a preferred embodiment the Al₂O₃ flakes of the present invention are α-Al₂O₃ flakes.

The Al₂O₃ flakes according to the invention can be prepared by methods known per se, as described in the literature.

In a preferred embodiment the Al₂O₃ flakes are prepared starting from an aqueous aluminum salt solution by precipitation with an aqueous alkali carbonate solution. An alkali metal salt like sodium or potassium sulfate and phosphoric acid or phosphate as well as a dopant like a titanium compound are added to the starting solution. The precipitation step is followed by drying (evaporation, dehydration by heating) and molten salt treatment including the following steps:

The Al₂O₃ flakes can be prepared by methods known per se, as described in the literature.

In a preferred embodiment the Al₂O₃ flakes are prepared starting from an aqueous aluminum salt solution by precipitation with an aqueous alkali carbonate solution. An alkali metal salt like sodium or potassium sulfate and phosphoric acid or and phosphate as and optionally at least one dopant, for example a titanium, zirconium, silica, indium, tin, zinc or indium compound are added to the starting solution. The precipitation step is followed by drying (evaporation, dehydration by heating), and molten salt treatment including the following steps:

-   -   (1) Preparation of an aqueous solution or a slurry of at least         one water-soluble and/or insoluble aluminum salt,     -   (2) Adding an alkali solution to the aluminum salt solution to         precipitate aluminum hydroxide particles, and adding a         phosphorous compound and optionally at least one dopant to the         aqueous solution before, during or after the precipitation,     -   (3) Evaporation of the water, followed by drying of the         precipitated product of step (2) to form the dried form of         alumina containing particle and alkali salt,     -   (4) Calcination, preferably at temperatures of 900-1400° C., for         0.5-10 h, preferably 1-6 h, of the dried form obtained in         step (3) to obtain Al₂O₃ flakes in the molten salt,     -   (5) Removing the water soluble part of the calcined material         obtained in step (4),     -   (6) Adjust the particle size and thickness, for example by         sieving, milling and/or sedimentation.

Examples for suitable aluminum salts are aluminum sulfate, aluminum chloride, aluminum nitrate, poly aluminum chloride, aluminum hydroxide, boehmite, basic aluminum sulfate and combinations thereof.

Examples for suitable alkali metal salts which act as mineralizer, include sodium sulfate, potassium sulfate, lithium sulfate, magnesium sulfate, sodium chloride and potassium chloride.

The phosphoric compound is preferably selected from phosphoric acid, phosphates, diphosphoric acid, sodium phosphate, ammonium phosphate dibasic and potassium phosphate. The amount of one or more phosphorous compound(s) is preferably 0.05-2 wt. % based on the alumina flakes.

The preferred example of the pH controlling agent for the precipitation is ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate and combinations thereof.

To control the particle size, thickness, optical properties and/or surface morphology it could be helpful to add one or more dopants in amounts of 0.01-5 wt. % based on the Al₂O₃ flake.

The dopant is preferably selected from the following group of compounds: TiO₂, ZrO₂, SiO₂, In₂O₃, SnO₂, ZnO and combinations thereof.

In a preferred embodiment the dopant is TiO₂, preferably used in amounts of 0.05-3 wt. % based on the Al₂O₃ flakes.

The Al₂O₃ flakes according to the present invention are highly suitable as substrate in the preparation of effect pigments. To this end, they are preferably coated with at least one high refractive index layer, like at least one layer of a metal oxide, such as, for example, TiO₂, ZrO₂, SnO₂, ZnO, Ce₂O₃, Fe₂O₃, Fe₃O₄, FeTiO₅, Cr₂O₃, CoO, Co₃O₄, VO₂, V₂O₃, NiO, furthermore of titanium suboxides (TiO₂ partially reduced with oxidation states from <4 to 2, such as the lower oxides Ti₃O₅, Ti₂O₃, TiO), titanium oxynitrides, FeO(OH), thin semitransparent metal layer, for example comprising Al, Fe, Cr, Ag, Au, Pt or Pd, or combinations thereof. The TiO₂ layer may be in the rutile or anatase modification. In general, the highest quality and gloss and at the same time the most stable effect pigments are obtained when the TiO₂ is in the rutile modification. In order to obtain the rutile modification, an additive can be used which is able to direct the TiO₂ into the rutile modification. Useful rutile directors such as tin dioxide are disclosed in the U.S. Pat. No. 4,038,099 and U.S. Pat. No. 5,433,779 and EP 0 271 767. Preferred effect pigments based on Al₂O₃ flakes are coated with one or more layers of metal oxides, preferably with one metal-oxide layer only, in particular with TiO₂, Fe₂O₃, Fe₃O₄, SnO₂, ZrO₂ or Cr₂O₃. Especially preferred are Al₂O₃ flakes coated with TiO₂ or Fe₂O₃.

The thickness of each high-refractive-index layer depends on the desired interference color. The thickness of each layer on the surface of the Al₂O₃ flakes is preferably 20-400 nm, preferably 30-300 nm, in particular 30-200 nm.

The number of layers on the surface of the Al₂O₃ flakes is preferably one or two, furthermore three, four, five, six or seven layers.

In particular, interference packages consisting of high- and low-refractive-index layers on the surface of the Al₂O₃ flakes result in effect pigments having increased gloss and a further increased interference color or color flop.

Suitable colorless low-refractive-index materials for coating are preferably metal oxides or the corresponding oxide hydrates, such as, for example, SiO₂, Al₂O₃, AlO(OH), B₂O₃, compounds such as MgF₂ or a mixture of the said metal oxides.

In case of multilayers applied on the surface of the Al₂O₃ flakes the interference system is, in particular, a TiO₂—SiO₂—TiO₂ layer sequence.

Furthermore, the effect pigments according to the invention may also have a semitransparent metal layer as outer layer. Coatings of this type are known, for example, from DE 38 25 702 A1. The metal layers are preferably chromium or aluminum layers having layer thicknesses of 5-25 nm.

Al₂O₃ flakes can also be coated with one or more layers of a metal or metal alloy selected e.g. from chromium, nickel, silver, bismuth, copper, tin, or hastalloy. Al₂O₃ flakes coated with a metal sulfide are coated with sulfides e.g. of tungsten, molybdenum, cerium, lanthanum or rare earth elements.

Furthermore, the effect pigments based on Al₂O₃ flakes can be finally coated with an organic dye as a top coat, preferably with Prussian Blue or Carmine Red.

Particularly preferred effect pigments based on the Al₂O₃ flakes according to the invention have the following layer sequence(s):

Al₂O₃ flake+TiO₂

Al₂O₃ flake+TiO₂/Fe₂O₃

Al₂O₃ flake+Fe₂O₃

Al₂O₃ flake+TiO₂+Fe₂O₃

Al₂O₃ flake+TiO₂+Fe₃O₄

Al₂O₃ flake+TiO₂+SiO₂+TiO₂

Al₂O₃ flake+Fe₂O₃+SiO₂+TiO₂

Al₂O₃ flake+TiO₂/Fe₂O₃+SiO₂+TiO₂

Al₂O₃ flake+TiO₂+SiO₂+TiO₂/Fe₂O₃

Al₂O₃ flake+TiO₂+SiO₂

Al₂O₃ flake+TiO₂+SiO₂/Al₂O₃

Al₂O₃ flake+TiO₂+Al₂O₃

Al₂O₃ flake+SnO₂

Al₂O₃ flake+SnO₂+TiO₂

Al₂O₃ flake+SnO₂+Fe₂O₃

Al₂O₃ flake+SiO₂

Al₂O₃ flake+SiO₂+TiO₂

Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃

Al₂O₃ flake+SiO₂+Fe₂O₃

Al₂O₃ flake+SiO₂+TiO₂+Fe₂O₃

Al₂O₃ flake+SiO₂+TiO₂+Fe₃O₄

Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂

Al₂O₃ flake+SiO₂+Fe₂O₃+SiO₂+TiO₂

Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃+SiO₂+TiO₂

Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂/Fe₂O₃

Al₂O₃ flake+SiO₂+TiO₂+SiO₂

Al₂O₃ flake+SiO₂+TiO₂+SiO₂/Al₂O₃

Al₂O₃ flake+SiO₂+TiO₂+Al₂O₃

Al₂O₃ flake+TiO₂+Prussian Blue

Al₂O₃ flake+TiO₂+Carmine Red

The TiO₂ layer(s) in the preferred embodiments mentioned above can be in the rutile or anatase modification. The Al₂O₃ flakes mentioned above in the preferred embodiments can be doped or undoped.

In this application, the term “coating” or “layer” is taken to mean the complete enveloping of the Al₂O₃ flakes according to the invention.

The effect pigments based on doped or undoped Al₂O₃ flakes preferably consist of 40-90 wt. % of Al₂O₃ flakes and 10-60 wt. % of the coating based on the total pigment.

The Al₂O₃ flakes can be coated by wet chemical coating, by CVD or PVD processes.

The coating of the α-Al₂O₃ flakes with one or more layers, preferably one or more metal oxide layers, is preferably carried out by wet-chemical methods, it being possible to use the wet-chemical coating methods developed for the preparation of pearlescent pigments. Methods of this type are described, for example, in DE 14 67 468, DE 19 59 988, DE 20 09 566, DE 22 14 545, DE 22 15 191, DE 22 44 298, DE 23 13 331, DE 15 22 572, DE 31 37 808, DE 31 37 809, DE 31 51 343, DE 31 51 354, DE 31 51 355, DE 32 11 602, DE 32 35 017 or also in further patent documents and other publications known to the person skilled in the art.

In the case of wet coating, the Al₂O₃ flakes are suspended in water, and one or more hydrolysable metal salts are added at a pH which is suitable for hydrolysis, which is selected in such a way that the metal oxides or metal-oxide hydrates are precipitated directly onto the flakes without secondary precipitations occurring. The pH is usually kept constant by simultaneous metered addition of a base and/or acid. The pigments are subsequently separated off, washed and dried at 50-150° C. for 6-18 h and calcined for 0.5-3 h, where the calcination temperature can be optimised with respect to the respective coating present. In general, the calcination temperatures are 500-1000° C., preferably 600-900° C. If desired, the pigments can be separated off after application of individual coatings, dried and optionally calcined and then re-suspended again for the application of the further layers.

The application of a SiO₂ layer to the Al₂O₃ flake and/or to the already coated Al₂O₃ flake is generally carried out by addition of a potassium or sodium water-glass solution at a suitable pH.

Furthermore, the coating can also be carried out in a fluidised-bed reactor by gas-phase coating, it being possible to use, for example, the methods proposed in EP 0 045 851 and EP 0 106 235 for the preparation of pearlescent pigments correspondingly.

The hue and chroma of the effect pigment based on Al₂O₃ flakes according to the invention can be varied in very broad limits through the different choice of the coating amounts or the layer thicknesses resulting therefrom. Fine tuning for a certain hue and chroma can be achieved beyond the pure choice of amount by approaching the desired color under visual or measurement technology control.

In order to increase the light, water and weather stability, it is frequently advisable, depending on the area of application, to subject the finished pigment to post-coating or post-treatment. Suitable post-coatings or post-treatments are, for example, the processes described in DE 22 15 191 C2, DE-A 31 51 354, DE-A 32 35 017 or DE-A 33 34 598. This post-coating further increases the chemical and photochemical stability or simplifies the handling of the pigment, in particular the incorporation into various media. In order to improve the weatherability, dispersibility and/or compatibility with the user media, it is possible, for example, for functional coatings of Al₂O₃ or ZrO₂ or mixtures thereof to be applied to the pigment surface. Furthermore, organic post-coatings are possible, for example with silanes, as described, for example, in EP 0090259, EP 0 634 459, WO 99/57204, WO 96/32446, WO 99/57204, U.S. Pat. No. 5,759,255, U.S. Pat. No. 5,571,851, WO 01/92425 or in J. J. Ponjeé, Philips Technical Review, Vol. 44, No. 3, 81 ff. and P. H. Harding J. C. Berg, J. Adhesion Sci. Technol. Vol. 11 No. 4, pp. 471-493.

In accordance with the present invention, an effect pigment based on Al₂O₃ flakes having the desired size distribution has been found useful in all types of compositions, including plastics, cosmetics, and, in particular in automotive paints.

The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes according to the invention are compatible with a multiplicity of color systems, preferably from the area of paints, automotive coatings, industrial coatings, and printing inks and cosmetic formulations. For the preparation of printing inks for, for example, gravure printing, flexographic printing, offset printing and offset overvarnishing, a multiplicity of binders, in particular water-soluble grades, as sold, for example, by BASF, Marabu, Pröll, Sericol, Hartmann, Gebr. Schmidt, Sicpa, Aarberg, Siegberg, GSB-Wahl, Follmann, Ruco or Coates Screen INKS GmbH, is suitable. The printing inks can be water-based or solvent-based. The Al₂O₃ flakes and the effect pigments according to the invention are furthermore also suitable for the laser marking of paper and plastics and for applications in the agricultural sector, for example for greenhouse sheeting, and, for example, for the coloring of tent awnings.

It goes without saying that, for the various applications, the coated and uncoated Al₂O₃ flakes according to the present invention can also advantageously be used in blends with organic dyes, organic pigments or other pigments, such as, for example, transparent and opaque white, colored and black pigments, and with flake-form iron oxides, organic pigments, holographic pigments, LCPs (liquid crystal polymers) and conventional transparent, colored and black luster pigments based on metal oxide-coated mica and SiO₂ flakes, etc. The pigments according to the invention can be mixed in any ratio with commercially available pigments and fillers.

Fillers which may be mentioned are, for example, natural and synthetic mica, nylon powder, pure or filled melamine resins, talc, SiO₂, glasses, kaolin, oxides or hydroxides of aluminum, magnesium, calcium or zinc, BiOCl, barium sulfate, calcium sulfate, calcium carbonate, magnesium carbonate, carbon, and physical or chemical combinations of these substances. There are no restrictions regarding the particle shape of the filler. It can be, for example, flake-form, spherical or needle-shaped in accordance with requirements.

The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes according to the invention are simple and easy to handle. The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes can be incorporated into the system in which it is used by simple stirring. Laborious milling and dispersing of the Al₂O₃ flakes and the effect pigments is not necessary.

The Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes according to the invention can be used for pigmenting coating materials, printing inks, plastics, agricultural films, button pastes, for the coating of seed, for the coloring of food, coatings of medicaments or cosmetic formulations. The concentration of the Al₂O₃ flakes and the effect pigments in the system in which it is to be used for pigmenting is generally between 0.01 and 50% by weight, preferably between 0.1 and 5% by weight, based on the overall solids content of the system. This concentration is generally dependent on the specific application.

Plastics containing the Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes according to the invention in amounts of 0.1 to 50% by weight, in particular from 0.5 to 7% by weight, are frequently notable for a particular gloss and shimmer effect.

In the coating sector, especially in automotive coating and automotive finishing, the effect pigments based on Al₂O₃ flakes according to the invention are employed in amounts of 0.5-10% by weight.

In the coating material, the Al₂O₃ flakes and the effect pigments based on Al₂O₃ flakes according to the invention have the advantage that the desired color and gloss is obtained by a single-layer coating (one-coat systems or as a base coat in a two-coat system).

In the pigmentation of binder systems, for example for paints and printing inks for intaglio, offset or screen printing, the effect pigments based on Al₂O₃ flakes with Stapa®-aluminum and gold bronze pastes from Eckart GmbH have proven particularly suitable. The effect pigment is incorporated into the printing ink in amounts of 2-50% by weight, preferably 5-30% by weight and, in particular, 8-15% by weight. The printing inks containing the effect pigment according to the invention in combination with a metal effect pigment exhibits purer hues and is of improved printability owing to the good viscosity values.

The invention likewise provides pigment preparations containing coated or uncoated Al₂O₃ flakes according to the present invention and further effect pigments, binders and, if desired, additives, the said preparations being in the form of substantially solvent-free, free-flowing granules. Such granules contain up to 95% by weight of the Al₂O₃ flakes or the effect pigments according to the invention. A pigment preparation in which the effect pigment based on Al₂O₃ flakes of the invention is pasted up with a binder and with water and/or an organic solvent, with or without additives, and the paste is subsequently dried and brought into a compact particulate form, e.g. granules, pellets, briquettes, a masterbatch or tablets, is particularly suitable as a precursor for printing inks.

The invention thus also relates to the use of the coated (=effect pigments) or uncoated Al₂O₃ flakes in formulations from the areas of paints, coatings, automobile coatings, automotive finishing, industrial coatings, paints, powder coatings, printing inks, security printing inks, plastics, ceramic materials, cosmetics. The coated and uncoated Al₂O₃ flakes can furthermore be employed in glasses, in paper, in paper coating, in toners for electrophotographic printing processes, in seed, in greenhouse sheeting and tarpaulins, in thermally conductive, self-supporting, electrically insulating, flexible sheets for the insulation of machines or devices, as absorber in the laser marking of paper and plastics, as absorber in the laser welding of plastics, in pigment pastes with water, organic and/or aqueous solvents, in pigment preparations and dry preparations, such as, for example, granules, for example in clear coats in the industrial and automobile sectors, in sunscreens, as filler, in particular in automobile coatings and automotive finishing.

All percentage data in this application are per cent by weight, unless indicated otherwise.

The following examples are intended to explain the invention in greater detail, but without restricting it. Above and below, all percentages are per cent by weight.

EXAMPLES Comparative Example 1 Example 2 of U.S. Pat. No. 5,702,519

In 300 ml of deionized water are dissolved 111.9 g of aluminum sulfate 18-hydrate, 57.3 g of anhydrous sodium sulfate, and 46.9 g of potassium sulfate by heating above 60° C. To the resulting solution is added 1.0 g of 34.4% solution of titanyl sulfate. The resulting solution is designated as the aqueous solution (a).

In 150 ml of deionized water are 0.45 g of sodium tertiary phosphate 12-hydrate and 54.0 g of sodium carbonate. The resulting solution is designated as the aqueous solution (b).

The aqueous solution (b) is added with stirring to the aqueous solution (a) kept at about 60° C. Stirring is continued for 15 minutes. The resulting mixture of the two solutions is a gel. This gel is evaporated to dryness, and the dried product is heated at 1200° C. for 5 hours. Water is added to the heated product to dissolve free sulfate. Insoluble solids are filtered off, washed with water, and finally dried. The obtained alumina flake is examined by X-ray diffractometry. The diffraction pattern has only peaks attributed to corundum structure (α-alumina structure).

D₅₀ is 13.0 μm and D₉₀ is 22.0 μm and the thickness is 200 nm.

Example 1 Preparation of the Al₂O₃ Flakes

74.6 g of aluminum sulfate 18-hydrate, 5.7 g of transition alumina (C10W: Nippon Light Metal Co., Ltd.), 57.3 g of anhydrous sodium sulfate and 46.9 g of potassium sulfate are dissolved in 450 ml of deionized water by heating above 60° C. To the resulting solution is added 1.2 g of 34.4 solution of titanyl sulfate. The resulting solution is designated as the aqueous solution (a).

In 300 ml of deionized water are 0.010 g of sodium tertiary phosphate 12-hydrate and 55.0 g of sodium carbonate. The resulting solution is designated as the aqueous solution (b).

The aqueous solution (b) is added with stirring to the aqueous solution (a) kept at about 60° C. Stirring is continued for 30 minutes. The resulting mixture of the two solutions is a slurry. This slurry is evaporated to dryness, and the dried product is heated at 1230° C. for 4.5 hours. Water is added to the heated product to dissolve free sulfate. Insoluble solids are filtered off, washed with water, and filtered by sieve with 25 micron opening. And finally it was dried. The obtained alumina flake is examined by X-ray diffractometry. The diffraction pattern have only peaks attributed to corundum structure (α-alumina structure).

D₅₀ is 19.6 μm, D₉₀ is 38.8 μm and the average thickness is 700 nm.

Comparative Example 1.1 Coating of Al₂O₃ Flakes

20 g of alumina flakes of Comparative Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 65° C.) is added a solution containing 125 g of TiCl₄ per liter. Simultaneously a 10 solution of NaOH was added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a silvery color. The suspending solids are filtered off, washed with water and dried. Finally, the dried solids are calcined at 850° C. for 30 minutes to give a whitish and a little glossy pearlescent pigment.

Example 1.1 Coating of Al₂O₃ Flakes

20 g of alumina flakes of Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 65° C.) is added a solution containing 125 g of TiCl₄ per liter. Simultaneously a 10% solution of NaOH was added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a silvery color. The suspending solids are filtered off, washed with water and dried. Finally, the dried solids are calcined at 850° C. for 30 minutes to give a highly whitish and glossy pearlescent pigment with a strong glitter effect.

Comparative Example 1.2 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 75° C.) are added a solution containing 300 g of FeCl₃ per liter. Simultaneously a 10 solution of NaOH was added to keep the pH at 3.0. The addition of the FeCl₃ solution was stopped when the resulting product takes on a most reddish color. The suspending solids are filtered off, washed with water, and dried. Finally, the dried solids are calcined at 800° C. for 30 minutes to give a moderate luster and brownish red colored pearlescent pigment.

Example 1.2 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 75° C.) are added a solution containing 300 g of FeCl₃ per liter. Simultaneously a 10% solution of NaOH was added to keep the pH at 3.0. The addition of the FeCl₃ solution was stopped when the resulting product takes on a most reddish color. The suspending solids are filtered off, washed with water, and dried. Finally, the dried solids are calcined at 800° C. for 30 minutes to give a high luster and pure red colored pearlescent pigment with a strong glitter effect.

Comparative Example 1.3 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 65° C.) is added a solution containing 50 g of SnCl₄ per liter. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1 until the total adding volume of SnCl₄ solution become 17 ml. Next the solution containing 125 g of TiCl₄ per liter was added to the resulting suspension. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a silvery color. The suspending solids are filtered off, washed with water, and dried. Finally, the dried solids are calcined at 850° C. for 30 minutes to give a whitish and a little glossy pearlescent pigment.

Example 1.3 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 65° C.) is added a solution containing 50 g of SnCl₄ per liter. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1 until the total adding volume of SnCl₄ solution become 17 ml. Next the solution containing 125 g of TiCl₄ per liter was added to the resulting suspension. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a silvery color. The suspending solids are filtered off, washed with water, and dried. Finally, the dried solids are calcined at 850° C. for 30 minutes to give a highly whitish and highly glossy pearlescent pigment with a strong glitter effect. At the luster angle, the glossy appearance can be seen at a more wider angle compared to Comparative Example 1.3.

Comparative Example 1.4 Coating of Al₂O₃ Flakes

20 g alumina flakes of Comparative Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 65° C.) is added a solution containing 125 g of TiCl₄ per liter. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a yellowish color. Then a solution containing 50 g of Na₂SiO₃ per liter is added to the resulted suspension. Simultaneously a 10% solution of HCl is added to keep the pH at 7. Next a solution containing 125 g of TiCl₄ per liter is added to the resulted suspension. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a bluish color. The suspending solids are filtered off, washed with water, and dried. Finally, the dried solids are calcined at 850° C. for 30 minutes to give a blue-whitish and a little glossy pearlescent pigment.

Example 1.4 Coating of Al₂O₃ Flakes

20 g alumina flakes of Example 1 are suspended in 400 ml of deionized water. To the resulting suspension (kept at about 65° C.) is added a solution containing 125 g of TiCl₄ per liter. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a yellowish color. Then a solution containing 50 g of Na₂SiO₃ per liter is added to the resulted suspension. Simultaneously a 10% solution of HCl is added to keep the pH at 7. Next a solution containing 125 g of TiCl₄ per liter is added to the resulted suspension. Simultaneously a 10% solution of NaOH is added to keep the pH at 2.1. The addition of the TiCl₄ solution is stopped when the resulting product takes on a bluish color. The suspending solids are filtered off, washed with water, and dried. Finally, the dried solids are calcined at 850° C. for 30 minutes to give a highly bluish and highly glossy pearlescent pigment with a strong glitter effect. At the luster angle, the stronger bluish color and higher glossy appearance can be seen at a more wider angle compared to Comparative Example 1.4.

Measurements

Evaluation for Particle Size D₁₀, D₅₀ and D₉₀

D₁₀, D₅₀ and D₉₀ of alumina flake are evaluated by using Malvern MS2000.

Determination of the Thickness and Particle Size and the Thickness Distribution

0.01 g/l of the alumina flake slurry is prepared and 0.1 ml of this slurry is dropped onto a flat substrate like a silicon wafer. The substrate is dried and cut to adequate size. The substrate is set with almost vertically tilted angle on the base of SEM (Scanning electronic microscope) and the thickness of the alumina flake is determined.

The thickness of more than 100 alumina flakes is measured for the calculation of the thickness distribution. The standard deviation of the thickness is calculated with the Gaussian distribution equation.

Preparation for a Sprayed Panel

A base coat paint for automobiles is prepared according to the following formulation.

<Base coat system> Acrylic-melamine resin system “Acrydic ® 47-712”* 70 pbw “Superbekkamine ® G821-60”** 30 pbw Toluene 30 pbw Ethyl acetate 50 pbw *Acrylic resin from Dainippon Ink & Chemicals, Inc. **Melamine resin from Dainippon Ink & Chemicals, Inc.

<Base coat system> Acrylic-melamine resin system n-Butanol 110 pbw Solvesso ® #150  40 pbw

The above acrylic-melamine resin system (100 pbw) is incorporated with 20 pbw of the pearlescent pigment according to one of the above given Examples. The resulting compound is diluted with a thinner so that the resulting paint has an adequate consistency for spraying (12-15 seconds, for cup #4). This paint is applied to a substrate by spraying to form a base coat layer.

The base coated layer is coated further with a colorless top clear coat paint, which is prepared according to the following formulation.

<Top Clear Coat System>

<Top clear coat system> “Acrydic ® 47-712” 14 pbw  “Superbekkamine ® L117-60” 6 pbw Toluene 4 pbw MIBK 4 pbw Butyl cellosolve 3 pbw

The top clear coating is exposed to air at 40° C. for 30 minutes and then cured at 135° C. for 30 minutes.

Haze-gloss (BYK) is the analyzing equipment to evaluate luster and haze. In this patent application, the value of mirror gloss on 60° is measured by Haze-gloss and represents the value of the luster. Compared to the prior art the coated Al₂O₃ flakes show very high values for the luster. High luster values are necessary to achieve a good appearance in the applications. The haze values measured by this equipment are affected by spreading of the reflection angle. In this patent application, the wider spreading angle is important for the pearlescent appearance. The coated Al₂O₃ flakes according to the present invention show very high haze-gloss values. To differentiate the evaluation result, panels are used which are sprayed with a base coat before they are coated with a top clear coat for the haze-gloss measurement.

Wave-scan dual (BYK) is used as the analyzing equipment to measure the surface flatness of the samples. Wa value represents the cyclic flatness in the range of 0.1-0.3 mm. A smaller value in this patent application represents a flatter surface showing the advantages of the pigments according to the present invention.

Sprayed panels with a top clear coat are measured for Wa. Flatter surfaces have better finishing appearance.

BYK-mac is used to evaluate glitter effect. The SG value represents the glitter.

Wave-scan dual (BYK) is used to evaluate the finishing quality of paint.

The optical properties of the pearlescent pigments according to the above given examples are summarized in the following table:

TABLE 1 Optical TiO₂ Particle size Average properties of coated distribution of the thickness of TiO₂ coated Al₂O₃ Al₂O₃ Al₂O₃ flakes (μm) the Al₂O₃ flakes Al₂O₃ flakes flakes flakes D₁₀ D₅₀ D₉₀ (nm) SG Wa E1 E1.1 9.0 19.6 38.8 700 8.5 13 CE1 CE1.1 4.8 13.0 22.0 200 2 21

The entire disclosures of all applications, patents and publications, cited herein and of corresponding European Application No. 13002294.0, filed Apr. 30, 2013 are incorporated by reference herein. 

1. Al₂O₃ flakes having a thickness of ≧500 nm and a D₅₀-value of 15-30 μm and a D₉₀-value of 30-45 μm.
 2. Al₂O₃ flakes according to claim 1, characterised in that the Al₂O₃ flakes are α-alumina flakes.
 3. Al₂O₃ flakes according to claim 1, characterised in that the D₉₀-value is 30-40 μm.
 4. Al₂O₃ flakes according to claim 1, characterised in that they have a particle thickness of 550-1000 nm.
 5. Al₂O₃ flakes according to claim 1, characterised in that they are doped with TiO₂, ZrO₂, SiO₂, SnO₂, In₂O₃, ZnO and combinations thereof.
 6. Al₂O₃ flakes according to claim 5, characterised in that the amount of doping is 0.01-5% by weight based on the Al₂O₃ flake.
 7. Al₂O₃ flakes according to claim 5, characterised in that they are doped with TiO₂.
 8. Al₂O₃ flakes according to claim 1, characterised in that the Al₂O₃ flakes are coated with at least one layer of a metal oxide, mixtures of at least two metal oxides, metal, metal sulphide, titanium suboxide, titanium oxynitride, FeO(OH), metal alloys or rare earth compounds.
 9. Al₂O₃ flakes according to claim 1, characterised in that the Al₂O₃ flakes are coated with at least one layer of a metal oxide or a mixture of at least two metal oxides.
 10. Al₂O₃ flakes according to claim 1, characterised in that the Al₂O₃ flakes are coated with the following layer sequence: Al₂O₃ flake+TiO₂ Al₂O₃ flake+TiO₂/Fe₂O₃ Al₂O₃ flake+Fe₂O₃ Al₂O₃ flake+TiO₂+Fe₂O₃ Al₂O₃ flake+TiO₂+Fe₃O₄ Al₂O₃ flake+TiO₂+SiO₂+TiO₂ Al₂O₃ flake+Fe₂O₃+SiO₂+TiO₂ Al₂O₃ flake+TiO₂/Fe₂O₃+SiO₂+TiO₂ Al₂O₃ flake+TiO₂+SiO₂+TiO₂/Fe₂O₃ Al₂O₃ flake+TiO₂+SiO₂ Al₂O₃ flake+TiO₂+SiO₂/Al₂O₃ Al₂O₃ flake+TiO₂+Al₂O₃ Al₂O₃ flake+SnO₂ Al₂O₃ flake+SnO₂+TiO₂ Al₂O₃ flake+SnO₂+Fe₂O₃ Al₂O₃ flake+SiO₂ Al₂O₃ flake+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃ Al₂O₃ flake+SiO₂+Fe₂O₃ Al₂O₃ flake+SiO₂+TiO₂+Fe₂O₃ Al₂O₃ flake+SiO₂+TiO₂+Fe₃O₄ Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+Fe₂O₃+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+TiO₂/Fe₂O₃+SiO₂+TiO₂ Al₂O₃ flake+SiO₂+TiO₂+SiO₂+TiO₂/Fe₂O₃ Al₂O₃ flake+SiO₂+TiO₂+SiO₂ Al₂O₃ flake+SiO₂+TiO₂+SiO₂/Al₂O₃ Al₂O₃ flake+SiO₂+TiO₂+Al₂O₃ Al₂O₃ flake+TiO₂+Prussian Blue Al₂O₃ flake+TiO₂+Carmine Red
 11. Al₂O₃ flakes according to claim 1, characterized in that the Al₂O₃ flakes are coated with TiO₂ in the rutile or anatase modification.
 12. Al₂O₃ flakes according to claim 1, characterized in that the Al₂O₃ flakes are coated with TiO₂ in the rutile modification.
 13. Al₂O₃ flakes according to claim 8, characterized in that they consist of 40-90 wt. % of Al₂O₃ flakes and 10-60 wt. % of the coating based on the total pigment.
 14. Process for the preparation of Al₂O₃ flakes according to claim 1, characterized by the following steps: (1) Preparation of an aqueous solution or a slurry of at least one water-soluble and/or insoluble aluminum salt, (2) Adding an alkali solution to the aluminum salt solution to precipitate aluminum hydroxide particles, and adding a phosphorous compound and optionally at least one dopant to the aqueous solution before, during or after the precipitation, (3) Evaporation of the water, followed by drying of the precipitated product of step (2) to form the dried form of alumina containing particle and alkali salt, (4) Calcination of the dried form obtained in step (3) to obtain Al₂O₃ flakes in the molten salt, (5) Removing the water soluble part of the calcined material obtained in step (4), (6) Adjust the particle size and thickness.
 15. A composition comprising the Al₂O₃ flakes according to claim 1, as substrate for effect pigments, in formulations from the areas of paints, coatings, automobile coatings, automotive finishing, industrial coatings, paints, powder coatings, printing inks, security printing inks, plastics, ceramic materials, cosmetics, glasses, paper, paper coating, toners for electrophotographic printing processes, seeds, greenhouse sheeting and tarpaulins, thermally conductive, self-supporting, electrically insulating, flexible sheets for the insulation of machines or devices, as absorber in the laser marking of paper and plastics, absorber in the laser welding of plastics, pigment pastes with water, organic and/or aqueous solvents, in pigment preparations and dry preparations.
 16. Formulation containing Al₂O₃ flakes according to claim 1 in amounts of 0.01-95% by weight, based on the formulation as a whole.
 17. Formulation containing Al₂O₃ flakes according to claim 1, characterised in that it contains at least one component selected from the group of water, polyols, polar and nonpolar oils, fats, waxes, film formers, polymers, copolymers, surfactants, free-radical scavengers, antioxidants, stabilisers, odour enhancers, silicone oils, emulsifiers, solvents, preservatives, thickeners, rheological additives, fragrances, colorants, effect pigments, UV absorbers, surface-active assistants and/or cosmetic active compounds, fillers, binders, pearlescent pigments, color pigments and organic dyes. 